Many manufacturing processes require the automated loading and unloading of work pieces into and out of a processing apparatus. In the interest of reducing cost and increasing productivity, such movement of work pieces is often accomplished with the aid of a robotically controlled load and unload mechanism.
One example of such a manufacturing process is the planarization of a surface of a work piece, a process that finds application in the manufacture of many types of products such as semiconductor wafers, optical blanks, memory disks, and the like. Chemical mechanical planarization (CMP) is one accepted method for achieving a planar surface on such work pieces. The CMP method typically requires the work piece to be loaded into and mounted precisely on a carrier head in a manner such that the surface to be planarized is exposed. The exposed side of the work piece is then held against a polishing pad and relative motion is initiated between the work piece surface and the polishing pad in the presence of a polishing slurry. Typically the work pieces are processed in batches or lots that include a plurality of work pieces. For example, with the CMP processing of semiconductor wafers, each of the wafers in a lot must be sequentially loaded from a wafer cache onto the carrier head for planarization. Following the planarization, each wafer is unloaded from the carrier head and again placed in a wafer cache or is directly transferred to a subsequent processing apparatus such as a cleaning station.
Loading a work piece into a chemical mechanical planarization apparatus presents problems for conventional work piece handling mechanisms because of the nature of the CMP carrier head. The conventional CMP carrier head includes a flexible diaphragm against which the back surface (the surface that is not to be polished) is pressed. The flexible diaphragm is surrounded by an annular wear ring or retaining ring having an inner diameter only slightly greater than the diameter of the work piece to be polished. The diaphragm and the wear ring form a cavity into which the work piece must be loaded. To carry out the planarization operation, the work piece must be mounted against the diaphragm within the confines of the wear ring. In the CMP processing of a semiconductor wafer the recess into which the semiconductor wafer must be loaded has a depth on the order of the thickness of the wafer itself, or about 0.75 mm, and the clearance between the inner diameter of the wear ring and the outer diameter of the semiconductor wafer is typically less than 1 mm.
With many work pieces, and certainly with semiconductor device wafers, the surfaces of the work pieces can be easily damaged if the surfaces are contacted during the loading and unloading processes. Because of this, the loading and unloading should preferably be done in a manner such that only the edge of the work piece or, at most, the surface within a narrow distance from the edge is contacted during the process. With the CMP processing of semiconductor wafers this requirement is made even more significant by the current migration of the semiconductor industry from 200 mm to 300 mm wafers. As part of this change, the semiconductor industry has adopted new wafer-handling standards for 300 mm wafers that preclude all contact with the major portion of the surfaces of a wafer, and has tightened limitations on the extent of the wafer that may be contacted near the wafer edge. Thus no significant contact with the front surface of the wafer is permitted and even known vacuum type end-effectors, or other end-effectors that grip or touch the back surface of the wafer are not allowed. These requirements and restrictions place serious limitations on the mechanisms used to handle the wafers. In addition, 300 mm wafers are significantly heavier than 200 mm wafers, adding still more demands on the mechanical integrity, precision, and reliability of the load and unload mechanisms.
Other types of processing apparatus in addition to CMP apparatus also require a work piece to be loaded into a recessed space with a high degree of positional accuracy and without adversely contacting the surfaces of the work piece. Although there are existing load and unload mechanisms such as robotically controlled work piece wands to address such applications, such mechanisms are costly, have difficulty with large or heavy work pieces, and require frequent maintenance and calibration to retain the necessary positional accuracy. Accordingly, it is desirable to provide an improved work piece handling mechanism that can load work pieces into and unload work pieces from a work piece processing apparatus with a high degree of precision and without adversely contacting the critical surfaces of the work piece. In addition, it is desirable to provide an improved chemical mechanical planarization (CMP) apparatus that includes a precision load and unload mechanism. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.